First one would be where some text gets displayed on a blank html page which includes Rich Text Markup Language (does such a term exists?). The user would then be able to save the page as an .rtf file and open it on their own .rtf-editor application (eg WordPad). This might not work because saving an html page as a file would include its own markup text as well as what gets displayed on the screen for the rich text format.

The second would be to have a WordPad-type text editor embedded in a page, much like the "reply" or "thread" generating editors used in this forum. In this case you'd need to create buttons and drop-downs, connect them to the text area using JS, perhaps create your own markup language that gets parsed to HTML when viewing the post, etc.

Either case sounds like a hefty project. You may have better luck seeing if you can get / purchase a text-editor that has already been generated. For example, this site uses vbulletin: